Documentary audio editing

I made a post a few weeks back about a similar subject (documentary editing), but there is one small thing I am having trouble grasping and thought you guys may be able to give me to clarity on this.

When a new speaker or topic is introduced, the audio will be brought in before the picture, then cut to the speaker and then back to b roll to illustrate the point further.

Firstly, I was wondering if there was any other creative reasons you have come across from personal experience to bring in the audio first (apart from cutting around mistakes, or just getting it to look and feel right?

Secondly,Whats confusing me is ive seen documentaries where the speaker is introduced first to introduce the subject and then it will cut to B roll to illustrate the point further.

I was thinking along the lines that maybe for using audio while the visuals are on the previous point (and possibly creating a link) is a nice way of making everything flow into one as opposed to point to point to point.

I understand this is a more of a stylistic thing and a very broad question, but why one way over the other?

Many times a first time speaker begins with something like...
"well, uuuhm, you know, the first time I saw Bob, uhm, you know, I was, well, surprised."
Fixed in post, this quickly becomes "The first time I saw Bob, I was surprised..." Cutaways hide these edits. 9 times out of ten, this is why you catch it as a split edit. Also, a viewer will always rather see Bob here than some dude babblin' about Bob. It's a natural fit to cover with Bob.
There are no rules here. That's what is great about documentaries. You get to make em up on the fly. In the end, making the best viewable product is the art. That quite often means making a nervous dude comfy in post.

L-cuts are dramatic in and of themselves, and, at times, they become a cliche'. As in those comedy bits where the guy is saying "no way you will EVER see me in (name of place)...." and then of course, the next cut reveals him saying that in the very place he was never going to ever allow himself to be.

Yes, you can use an L-cut dramatically and artistically, as well. I would say as a rule, make the cut that is least noticable, a cut that does not stand out and scream: "oh, isn't this a cute trick?". Because as cute as it may be, if it takes you out of the story, it is the wrong cut to use.